Method for coating graphite filaments with refractory metal carbides

ABSTRACT

FINE GRAPHITE FILAMENTS FOR USE AS REINFORCEMENTS IN METALS AND ALLOYS ARE COATED WITH A THIN LAYER OF A REFRACTORY METAL CARBIDE BY HEATING THE FILAMENTS IN A LOWMELTING METAL CONTAINING A SMALL AMOUNT OF A REFRACTORY METAL.

United States Patent Oflice 3,823,029 Patented July 9, 1974 3,823,029METHOD FOR COATING GRAPHITE FILAMENTS WITH REFRACTORY METAL CARBIDESMoinuddin S. Rashid, Ames, Iowa, assignor to the United States ofAmerica as represented by the United States Atomic Energy Commission NoDrawing. Filed Aug. 1, 1972, Ser. No. 276,989 Int. Cl. B44d 5/12 US. Cl.117-118 6 Claims ABSTRACT OF THE DISCLOSURE Fine graphite filaments foruse as reinforcements in metals and alloys are coated with a thin layerof a refractory metal carbide by heating the filaments in a lowmeltingmetal containing a small amount of a refractory metal.

CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein wasmade in the course of, or under, a contract with the United StatesAtomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to a method forcoating fine graphite filaments. More specifically, this inventionrelates to a method for coating fine graphite filaments with arefractory metal carbide.

The reinforcement of metals and alloys with filaments having qualitiesof low density, high strength and high modulus promises a new class ofadvanced engineering materials. Presently available filaments can bedivided into several groups, such as amorphous, single crystal,multiphase and polycrystalline. Each filament type has its advantagesand limitations and the selection of a given filament for a givenmetal-matrix composite is based partially on the conditions imposed bythe application.

Graphite filaments, which are polycrystalline, appear to be promisingfor strengthening metals for use at elevated temperatures. The averagestrengths of commercially available graphite filaments range as high as400,000 p.s.i., although experimental grades of graphite have beenreported to have tensile strengths of 500,000 p.s.i.

Besides their superior strength at room temperature, graphite filamentshave the ability to withstand extremely high temperatures in aprotective atmosphere without loss of stiffness or strength. Thisproperty makes aphite filaments far superior to most filaments asreinforcements in composite materials for use at elevated temperatures.Also, graphite filaments have a small diameter of about 8 to 10 whichmakes it possible to fabricate them using glassfiber technology, and alow density, which ensures that the strength/density and modulus/densityratios will be high.

Although the outlook for graphite is promising, the material has onemajor drawback which has limited its use in metal-matrix composites. Ithas a very high reactivity with most metals at elevated temperatures.This shortcoming must be eliminated or reduced before graphite will besuitable for use as a reinforcing filament. One possible solution is todevelop a suitable protective coating which will prevent direct contactbetween the filament and matrix.

Theoretically, the coating can be formed by deposition of the metal onthe surface of the filaments and interdiffusion with the graphite atelevated temperatures. However, most established processes cannot beused successa fully with filaments because of the stringent conditionswhich are imposed by their small size and the required high quality ofthe coating. To ensure against failure, the

coating must be very adherent, continuous and of a uniform thickness.Also, it must not be more than a few microns thick. A thick layer ofcarbide could be a source of weakness in the composite material and,since presently available graphite filaments are only 8,44 or less indiameter, the formation of a thick carbide coating will lead to aconsiderable reduction in the diameter of the filament.

SUMMARY OF THE INVENTION A method has been developed for forming thinrefractory metal coatings on graphite filaments having the desiredproperties enumerated above. In accordance with this invention, the finegraphite filaments can be coated with a refractory metal carbide bycontacting the filaments with a low-melting-point metal containing /2 to5 weight percent (w/o) of a refractory metal to form a charge, heatingthe charge to a temperature of about 1400 C. in an inert atmosphere fora short period of time, whereby a coating of a refractory metal carbideis formed on the surface of the filaments, cooling the charge andseparating the coated filaments from the low-melting-pointmetal-refractory metal alloy.

It is one object of this invention to provide fine graphite filamentssuitable for reinforcing metal composites.

It is another object of this invention to provide fine graphitefilaments suitable for reinforcing metals in a high-temperatureenvironment.

It is another object of this invention to provide fine graphitefilaments having a refractory metal carbide coatmg.

It is another object of this invention to provide a method for coatinggraphite filaments with a refractory metal carbide.

It is still another object of this invention to provide a method forcoating graphite filaments with a refractory metal carbide for use inthe reinforcement of metals and alloys.

It is a further object of this invention to provide a method for coatinggarphite filaments with a refractory metal carbide which is even,continuous and of uniform thickness.

Finally, it is the object of this invention to provide a method forcoating graphite filaments with a refractory metal carbide by heatingthe filaments in the presence of a low-melting-point metal containing arefractory metal.

DESCRIPTION OF THE PREFERRED EMBODIMENT These and other objects may bemet by contacting the graphite filaments to be coated with alow-melting-point metal such as tin containing about 1 w/o of arefractory metal such as niobium or tantalum to prepare a charge,heating the charge in an inert atmosphere to a temperature of about 1400C., maintaining the charge at this temperature for 2 to 3 minutes, untilthe refractory metal coating is formed, Cooling the charge andseparating the coated filaments from the metal.

Although tin is the preferred low-melting-point metal, otherlow-melting-point metals such as bismuth and lead are also satisfactory.

Although niobium and tantalum are the preferred refractory metals forthe process of this invention, other metals such as molybdenum,zirconium, titanium and hafnium should also provide a satisfactoryrefractory metal coating on the fine graphite filaments.

The concentration of refractory metal present in the low-melting-pointmetal may range from about /2 to S w/o, although /2 to 1 /2 w/o ispreferred and about 1 w/o is most preferred. Higher concentrations mayresult in a complete reaction between the refractory metal and thegraphite filament, thus completely consuming the graphite. The lowerconcentration permits improved control over the depth of the carbidecoating on the graphite filament. The ratio between the amount ofrefractory metal and the graphite filaments is not critical as long asthere is sufiicient refractory metal present to react with the graphiteto form the metal carbide.

Heating should take place under an inert gas such as helium or argon toprevent any undesirable reactions from taking place between the graphiteor refractory metal and the atmosphere.

Since the process of this invention is diffusion controlled, thethickness of the coating is controlled by carefully varying the time andtemperature conditions. In gen eral, it is believed that a 2,1. thickcoating on the fine graphite filaments will provide a sufficientdiffusion barrier to protect the remaining graphite filament, and theconditions given herein are directed toward providing a coating ofapproximately this thickness; however, it is obvious that, by varyingcertain parameters of this method thicker coatings can be applied shouldthey be necessary.

The temperature to which the charge is to be heated will depend upon themelting temperature of the low-meltingpoint metal and the amount ofrefractory metal which it contains. In general, it was found that atemperature of 1350 to 1450 C. was satisfactory for the compositionsdescribed herein, while a temperature of 1400 C. is preferred.

The length of time the charge remains at these temperatures is alsocritical to control the depth of the refractory metal carbide coating.In general, it was found that by maintaining the temperature at about1400 C. for 2 to 3 minutes before permitting the container to cool toroom temperature was sufiicient to obtain about a 2n coating ofrefractory metal carbide. In order to control the length of time thecharge was at the diffusion temperature, the charged crucible was heatedrapidly (less than minutes) to 1400 C.

Any crucible is satisfactory to practice the method of this inventionwhich will not react with the constituents. In general, a graphitecrucible was found to be satisfactory, although it is necessary toprepare a refractory metal carbide coating on the inner surface of thecrucible to prevent loss of the small amount of the refractory metalpresent by reaction with the crucible. This coating can be readilyapplied by filling the crucible with lowmelting-point metal containingseveral weight percent of refractory metal and heating the filledcrucible under an inert atmosphere to at least about 1400" C. for aperiod of time sufficient to react the refractory carbide with the innersurface of the graphite to form a refractory metal carbide coating onthe inner surface of the crucible.

The refractory-rnetal-carbide-coated graphite filaments may be separatedfrom the low-melting-point metal by methods known to those skilled inthe art. For example, the low-melting-point metal could be dissolved ina concentrated mineral acid or the charge could be heated in a vacuum toa temperature sufficiently high to evaporate the alloy, thus leaving therefractory-metal-carbidecoated graphite filaments.

EXAMPLE I Twenty-mesh tin powder and /z-inch-long segments of finegraphite filaments were stacked in alternate layers inside a graphitecrucible which had previously been coated with niobium carbide toprevent a depletion of the refractory metal by the walls of thecrucible. All filaments in the crucible were placed along the same axis.Approximately 1 w/o -325 mesh niobium powder was sprinkled on top of thetin and gradually worked into the crevices by vibrating the crucible,thereby forming the charge. The charged crucible was heated to about1400 C. under a helium atmosphere at a fast rate (less than 10 minutes)in a carbon resistor furnace and held at that temperature for 2 to 3minutes before cooling the charge to room temperature.

Electron-probe scans made on polished sections across thegraphite/carbide and carbide/tin interfaces indicated that the niobiumwas concentrated in the carbide coating and in a thin layer (reactionzone) adjacent to the carbide tin interface.

The metallographic examination also showed that the thickness of thecoating was uniform and that no cracks were present except when therewas a large pore in the graphite. These properties remained unchangedwhen the coating of Nb C was converted to NbC. Both Nb C and NbC wereextremely adherent to the graphite and did not chip off when the coatedsurface of the specimen was ground on a 600-mesh SiC polished wheel.

The metallographically polished specimens were also used in theelectron-probe microanalysis. Line scans for niobium were made acrossthe diameter of several filaments and it was determined that nomeasurable amount of niobium had diffused into the graphite filament andthat the niobium present was located in the carbide coating.

EXAMPLE II Tin powder and /-inch-long segments of graphite filament werestacked in alternate layers inside a graphite crucible which hadpreviously been coated with a tantalum carbide coating. Approximately 1w/o 325 mesh tantalum powder was sprinkled on top of the tin andgradually worked into the crevices by vibrating the crucible, therebyforming the charge. The charged crucible was heated to about 1400 C.under a helium atmosphere at a fast rate (less than 10 minutes) in acarbon resistor furnace and held at that temperature for 2 to 3 minutesbefore cooling the charge to room temperature.

The coated filaments were examined as described in Example I and werefound to have a uniform, continuous coating of tantalum carbide.

EXAMPLE III Five different commercially available graphite filamentswere coated with niobium and tantalum carbide by the method described inExamples I and II. They were:

A. Polyacrylonitrile-derived (PAN):

1. Hercules Inc., HM-S, high-modulus 2. Hercules, Inc., HT-S,high-strength 3. Morganite, Type I, untreated, high-modulus,polyacrylonitrile 4. Morganite, Type H, untreated, high-strength B.Rayon type:

1. Thornel-SO, PVA sizes 2 ply yarn The coated filaments were thenexamined as described in Example I. The coating on the Pan typefilaments which have a circular cross section and a smooth surface wasof a superior quality. It was continuous and uniform on more than of thefilaments. There were very few filaments which were either partially orirregularly coated. Also, the coating on all the filaments in an ingothad an almost identical thickness of approximately 2p.

The quality of the coatings on the Rayon type filaments does not appearto be regular as was the coating on the Pan type of filaments and thisis basically due to the diiference in the surfaces of the two types offilament. The crenulated edges on the Rayon type filaments cause thegraphite/carbon interface to be irregular. As the interface advances,the reduction in the size of the filaments as well as the surface of thecoating become crenulated. Although these coatings were crenulated, theywere still very continuous and the average coating thickness on most ofthe filaments in an ingot was uniform. However, due to their smallersize, a few of the filaments were completely converted to carbide. Theirregular nature of the coating has one additional disadvantage. Itcould cause the effect of notches on the surface which could lead to thedegradation of the tensile strength of the filament.

As can be seen from the examples, the method of this invention willsuccessfully produce an even, uniform refractory metal carbide coatingon most available types of fine graphite filaments.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of forming a thin refractory metal carbide coating ongraphite filaments comprising: contacting the filaments with alow-melting-point containing /2 to 5 weight percent of a refractorymetal to form a charge, said low-melting-point metal being selected fromthe group consisting of tin, bismuth and lead, and said refractory metalbeing selected from the group consisting of niobium, tantalum,molybdenum, zirconium, titanium and hafnium; heating the charge to atemperature of about 1350-1450" C. under an inert atmosphere andmaintaining this temperature for 2 to 3 minutes whereby a thin coatingof refractory metal carbide is formed on the filaments; cooling thecharge and separating the coated filaments from the low-melting-pointmetal and refractory metal.

2. The method of claim 1 wherein the low-meltingpoint metal containsabout 1 weight percent of a refractory metal and the charge is heated to1400 C.

3. The method of claim 2 wherein the low-meltingpoint metal is tin andthe refractory metal is niobium.

4. The method of claim 2 wherein the low-meltingpoint metal is tin andthe refractory metal is tantalum.

5. A method of forming a thin refractory metal carbide coating ongraphite filaments comprising: packing the filaments into a graphitecrucible, adding to the crucible and filaments a mixture of powdered tincontaining about 1 weight percent of a powdered refractory metalselected from the group consisting of niobium and tantalum to form acharge; heating the charge within a period of 10 minutes to about 1400C. under an inert atmosphere and maintaining this temperature for 2 to 3minutes whereby a thin coating of refractory metal carbide is formed onthe 9 filaments; cooling the charge and separating the coated filamentsfrom the tin and refractory metal.

6. The method of claim 5 comprising the additional step of forming ametal carbide coating on the graphite crucible before preparing thecharge to prevent depletion of the refractory metal in the charge.

References Cited UNITED STATES PATENTS 2,929,741 3/1960 Steinberg 29195A X 3,366,464 1/1968 Guichet et a1. 11721 X 3,369,920 2/1968 Bourdeau eta1. l17121 X WILLIAM D. MARTIN, Primary Examiner T. G. DAVIS, AssistantExaminer U.S. Cl. X.R.

